Peers who engage in pipeline compensation all know that what is the most afraid of choosing metal expansion joints? I am afraid that it will leak within two months of installation, or the pipe will be damaged by the wrong stiffness. This stiffness thing looks simple, but there are many practical ways. Today, let's make it clear how the stiffness of the metal expansion joint affects the selection and how to grasp that degree.
First, the definition of stiffness: it is not just a question of "hard or not"
Many people think that stiffness is "how hard it rebounds when pressed down", yes, but not entirely right. The stiffness of a metal expansion joint, professionally speaking, is the force required to produce unit deformation, and the unit is usually N/mm. To put it bluntly, how much effort it takes you to pull it 1mm, that's the stiffness value. But don't underestimate this number-it directly determines whether the expansion joint is "pliable" or "stubborn". The stiffness is too large, and it can't be pushed when the pipe expands thermally, and the stress is all accumulated on the pipe nozzle and equipment; The stiffness is too small, and it can't bear the internal pressure and self-weight, so the peaks and valleys will be scrapped as soon as they collapse.
Two days ago, I met a customer who chose a general-purpose corrugated expansion joint with low stiffness. As a result, it was used on the steam pipeline for two months, and the corrugated pipe bulged. The reason is that the rigidity is insufficient when the pressure fluctuates, and the bellows is unstable. Therefore, the greater the stiffness is not the better, nor the smaller the better, it has to be perfectly consistent with the working conditions.
2. Correlation between stiffness and performance: displacement, stress and fatigue life
Displacement capacity, stress level, fatigue life. You think about it, the smaller the stiffness, the greater the deformation under the same force, and the flexibility to absorb heat displacement is good. But at what cost? The stress cycle amplitude of bellows wall is large, and the fatigue life drops straight down. On the other hand, the stiffness is large and the displacement absorption capacity is poor, but its stress change per cycle is small, and its life may be long-provided that the compensation amount is sufficient.
High temperature axial type expansion joint, used in main steam pipeline of power plant, temperature above 550℃. At this time, the stiffness must be controlled tightly-too rigid, the pipe frame and steam turbine interface can't stand it; Too soft, the bellows can't hold up for years under high temperature creep. We generally calculate according to EJMA standard, and control the single wave displacement within half of the allowable value, so that the influence of stiffness on stress life is acceptable. Check the case of corrugated expansion joint used in power station industry in this station, and the fatigue test data in it are sticked to the actual working conditions.
3. Key factors affecting stiffness: wave height, wall thickness, number of layers and materials
These four points will change whoever touches them. The larger the wave height, the easier the bellows to deform and the lower the stiffness; The larger the wall thickness, the harder it must be, and the stiffness soars. What about the number of layers? Multi-layer thin-wall superposition, each layer shares the stress, the total stiffness is smaller than that of a single layer with the same thickness, but the flexibility is better. The material is even more critical-austenitic stainless steel 304 and 316L have similar elastic modulus, but the strength attenuation is different at high temperature, and the stiffness also changes. If you switch to Incoloy 800, the high-temperature stiffness retention rate is much higher than that of 304.
Two days ago, there was a desulfurization flue who wanted to use a non-metallic expansion joint. Later, there was high-temperature gas channeling under the working condition, so he used a large-diameter thick-walled expansion joint instead. Customers asked why not use the regular generic type? I said that the internal pressure of your flue is 0.3MPa, the diameter is 2 meters, and the wave height is not big. You can only rely on thickening the wall and increasing the number of layers to keep the stiffness, otherwise it will deflate as soon as it is pressurized. This is the typical insufficient stiffness, structural failure.
4. Difference of stiffness of expansion joints of different structural types-from axial type to pressure balance type
Different structures, stiffness difference heaven and earth. Single axial type (such as general corrugated expansion joint), the bellows directly bears axial force, and the stiffness is the axial stiffness of the bellows itself. The lateral expansion joint of compound hinge depends on two bellows plus hinge group to absorb the lateral displacement. The stiffness of a single bellows may not be high, but the lateral stiffness of the whole system is superimposed, so it must be recalculated according to the hinge constraint during design.
What about the pressure balance type? For example, straight pipe pressure balance expansion joint and curved pipe pressure balance expansion joint, they rely on two bellows facing each other to offset the blind plate force generated by internal pressure. The axial stiffness of this structure is actually the vector sum of the stiffness of the two bellows, and it is often much larger than that of the single axial type of the same diameter. If you think about it, with an extra set of bellows, the stiffness of course goes up. However, the advantage is that the force on the connecting pipe at the end of the equipment is small, which is suitable for sensitive equipment such as steam turbine and compressor.
There is also a single axial expansion joint with external pressure. The bellows is subjected to external pressure on the outside, which requires high stability. The wall thickness is usually one gear larger than that of the internal pressure type, so the axial stiffness is also larger. To sum up: Don't just look at the name of the product, you have to look at the structural principle. The stiffness of different models is completely different. The double hinge transverse expansion joint and straight pipe pressure balance expansion joint of this station have detailed parameters, which you can compare.
V. Stiffness Calculation and Selection Practice: Engineering Logic Behind the Formula
Stiffness calculation is not a slap on the head. The common formula comes from EJMA or GB/T 12777, and the core is:
Axial stiffness K = (π ·E·t³·n) / (6 · (1-ν ²) ·h0³) × correction factor
E is the elastic modulus, t is the wall thickness of a single layer (converted if there are more layers), h₀ is the wave height, and n is the wave number. Do you see that? Wall thickness is a cubic effect, and wave height is also cubic (in the denominator). Therefore, in the design stage, adjusting wave height and wall thickness are the two most effective knobs for adjusting stiffness.
But the light formula is useless, it has to be combined with engineering constraints. For example, metal corrugated expansion joints in cement industry usually have large dust and frequent temperature fluctuations. Customers require that the stiffness should not be too low, otherwise the bellows will deform unevenly after dust accumulation. We generally multiply the calculated stiffness by a safety factor of 1.2, and then balance it with the pipeline thrust. To put it bluntly, the formula is dead and the on-site conditions are alive. When selecting, you have to do the pipeline stress analysis again to see what the allowed thrust of the equipment interface is and the upper limit of stiffness required for reverse push.
By the way, don't be superstitious about "less stiffness is better". Two days ago, there was a case where a super-soft rubber compensator was selected (yes, this station also has a rubber compensator). As a result, the pipeline's own weight pressed the compensator into an "O" shape, and the medium could not flow. Therefore, the stiffness must match the arrangement of the pipe support, and the displacement cannot be just looked at.
6. Selection suggestions: how to match stiffness according to working conditions
- High temperature and high pressure occasions(For example, power station, petrochemical): high-temperature axial expansion joint or straight pipe pressure balance type is preferred. The stiffness is reduced according to the high-temperature elastic modulus of the material, and a margin needs to be left. It is recommended to do fatigue life accounting ≥1000 times.
- Large diameter low pressure(For example, behind desulfurization flue and flue gas baffle door): Use large-diameter thick-walled expansion joint or external pressure single axial type, and the wall thickness increases the stiffness to prevent internal pressure instability.
- Need to absorb large lateral displacements: Equipped with a compound hinge transverse expansion joint, the hinge limits the stiffness in one direction, and the other directions are flexible. Pay attention to the influence of hinge wear on stiffness and check it regularly.
- Vacuum or air cooling system: The stiffness of double hinge expansion joint of air-cooled island vacuum pipeline should take into account the vacuum pressure and thermal displacement of pipeline, and the wall thickness should not be too thin. The vacuum special hose of this site can also be referred to.
- Equipment with blind plate force restraint(such as steam turbine, compressor): Upper curved tube pressure balance expansion joint or double seal single axis circular baffle door are used in combination, although the stiffness is large, the connecting force is almost zero.
The stiffness of metal expansion joint is not an isolated parameter, it is entangled with material selection, structure, technology and installation. Don't just look at the stiffness value on the sample, but make a comprehensive judgment based on your pipe layout, bracket type, and media characteristics.